Hitherto, a multilayer ceramic capacitor has been produced by stacking electrode layers mainly composed of a metal powder and dielectric layers and subjecting the resulting stack to a firing step into one piece. Specifically, dielectric materials are mixed together to form a paste. The paste is formed into sheets. A conductive paste to be formed into electrode layers is applied to the dielectric sheets. The sheets are stacked and pressure-bonded. The resulting stack is sintered to combine the dielectric layers and the electrode layers into one piece, thereby producing a multilayer ceramic capacitor. Recent trends toward miniaturization and higher capacitance of multilayer ceramic capacitors have required electrode layers each having a smaller thickness. To produce such multilayer ceramic capacitors, metal powders for conductive pastes are required to have finer particles and high dispersibility.
Metal powders for conductive pastes used in multilayer ceramic capacitors are also required to have resistance to sintering. Sintering temperatures of metal powders used for conductive pastes are about 400° C. In contrast, sintering temperatures of dielectrics are about 1000° C. In firing steps for multilayer ceramic capacitors, both of dielectric layers and electrode layers need to be sintered. Thus, they are fired at sintering temperatures of dielectric layers that are sintered at higher temperatures. However, as described above, a difference in sintering shrinkage behavior between dielectric layers and electrode layers due to a difference in sintering behavior therebetween causes cracking in capacitors and a reduction in coverage. Thus, for the purpose of bringing the sintering shrinkage behavior of electrode layers closer to that of dielectric layers, fine dielectric particles are incorporated into electrode layers to inhibit the sintering of metal powders.
Fine dielectric particles incorporated into the electrode layers have at least two effects of inhibiting sintering during sintering: A first effect is one in which the fine dielectric particles inhibit contact between metal particles, necking between the metal particles, and sintering of the electrodes. A second effect is one in which the fine dielectric particles are deposited at the triple point of the metal particles (crystals) during the sintering of a metal powder, pin the metal particles, and inhibit the sintering of the electrodes. A more uniform dispersion of the fine dielectric particles in the metal powder further improves the effect of inhibiting the necking of the metal particles and the effect of pinning the metal particles, thereby inhibiting the sintering the metal powder. For this reason, a method for producing a metal powder containing fine dielectric particles uniformly dispersed therein is required.
Japanese Unexamined Patent Application Publication No. 2006-4675 (hereinafter, referred to as “Patent Document 1”) discloses a production method in which, for the purpose of bringing the thermal shrinkage characteristics of a Ni powder closer to those of dielectric ceramic layers and producing a conductive particle powder having excellent oxidation resistance and dispersibility in a conductive coating, an organic solvent in a slurry prepared by the addition of metal alkoxides 114 and 116 to a slurry of a Ni powder 112 dispersed in an organic solvent is evaporated and in which drying is performed to allow the alkoxides 114 and 116 to react during the drying (see FIG. 2).
However, in the production method described in Patent Document 1, a reaction occurs during the drying at a position other than the vicinity of the Ni powder 112 in a solution, in addition to the vicinity of the Ni powder 112, so that a metal compound that is not combined with the metal powder is formed. A metal powder 132 in which a metal and a complex metal compound are combined together is not efficiently produced. Furthermore, in the production method, the metal alkoxides 114 and 116 are used, and so the organic solvent must be used. It is not easy to handle the reaction mixture. It is necessary to take high-cost measures against explosions.
Japanese Patent No. 3475749 (hereinafter, referred to as “Patent Document 2”) discloses, for the purpose of effectively inhibiting the sintering of a Ni powder at a low temperature and producing a high-conductivity conductive film even at a small film thickness, a method in which a uniform solution containing a pyrolytic Ni compound and a pyrolytic compound (Ca, Ba, Ti, or Zr) to be converted into an oxide is prepared and heated at a decomposition temperature or higher in a reducing atmosphere. Specifically, a Ni powder including a complex oxide layer is produced at 1500° C. in a furnace by a spray pyrolysis method (see FIG. 3).
However, in the production method disclosed in Patent Document 2, heat treatment is performed at a high temperature during the production to allow the aggregation and coalescence of the Ni powder to proceed. Thus, the dispersibility of the complex oxide in the Ni powder is not sufficient.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-4675
Patent Document 2: Japanese Patent No. 3475749